1.

If A, B, C, D be any four points and E and F be the middle points of AC and BD respectively, then `A vec(B) + C vec(B) +C vec (D) + vec(AD)` is equal toA. `3 vec(EF)`B. `4vec(EF)`C. `4 vec(FE)`D. `3 vec(FE)`

Answer» Correct Answer - B
Since F is the middle point of BD. Therefore,
`A vec(B)+vec(AD)=2 vec(AF) " " ` …(i)
Similarly, we have
`vec(CB)+vec(AD)+2vec(CF) " " ` …(ii)
Adding (i) and (ii), we get
`A vec(B)+vec(AD)+C vec(B)+vec(CD)=2(vec(AF)+vec(CF))= -2(Fvec(A)+Fvec(C)) `
`rArr A vec(B) +vec(AD)+C vec(B)+vec(CD)= -2(2vec(FE))`
`rArr A vec(B)+vec(AD) +C vec B +vec(CD)=4vec(EF) " " [ because E " is the mid-point of " AC]`


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